CBCT apparatuses are known in the art and provide tomographic images of an anatomic portion by acquiring a sequence of bi-dimensional radiographic images during the rotation of a system that comprises an X-ray source and an X-ray detector around the anatomic part to be imaged.
A CBCT apparatus includes substantially: an X-ray source projecting a conic X-ray beam (unless it is subsequently collimated) through an object to be acquired; a bi-dimensional X-ray detector positioned so as to measure the intensity of radiation after passing through the object; a mechanical support on which said X-ray source and detector are fixed; a mechanical system allowing the rotation and the translation of said support around the object, so as to acquire radiographic images from different positions; an electronic system adapted to regulate and synchronize the functioning of the various components of the apparatus; and a computer or similar, adapted to allow the operator to control the functions of the apparatus, and to reconstruct and visualize the acquired images. There are substantially two kinds of such apparatuses on the market: a first kind where the patient stands or sits vertically during the acquisition, described e.g. in patent EP2018121B1 to Sirona; and a second kind where the patient lies on a table, described e.g. in patent IT1277796 to QR.
In a given CBCT apparatus, the maximum dimension of the volume to be reconstructed, or maximum FOV, is essentially determined by the geometry of the system, in terms of: dimensions of the sensitive area of the detector hit by X-ray, the distances between X-ray source and detector and the rotation axis of the system.
With the aim of obtaining CBCT apparatuses acquiring and reconstructing bigger and bigger volumes, the most direct solutions would lead to larger X-ray detectors, X-ray sources having an ampler angle of beam aperture, and/or an increase of the distance between X-ray source and object to be acquired. Generally speaking, each of these solutions involves significant disadvantages: e.g. economic disadvantages, in that the components are more expensive, or otherwise ergonomic disadvantages, linked to the need to increase the overall dimensions of the apparatus.
The technical problem of increasing the dimension of the maximum FOV without modifying the physical components of the system has already been addressed and solved in different ways, e.g.:
by offsetting the X-ray sensor, as described in U.S. Pat. No. 8,363,780 B2 to Carestream Health/Trophy;
by performing composite trajectories, as described in U.S. Pat. No. 8,300,762 B2 to Morita;
in the slightly different field of hospital bi-dimensional radiography, by summing two subsequent acquisitions of different portions of a long bone to obtain the image of the complete bone, as described in U.S. Pat. No. 7,555,100 B2 to Carestream Health, Inc.
The above solutions each have several drawbacks.
In regard to U.S. Pat. No. 8,363,780, as it appears clear from the description and the figures, by carrying out an offset of the sensor active area, problems arise with the intensity distribution of the radiation on the active surface, thus causing compensation actions to be performed, one of which includes the adjustment of the orientation of the generator as described in said document with reference to FIG. 3c. As illustrated, the effect of the compensation does not fully recover the lack of homogeneity of the distribution of the radiation along the active area of the sensor. Furthermore, there is a relation between the offset of the sensor's active area and the best orientation of the anode of the generator, so that this adjustment step has to be carried out each time an offset value is chosen.
Another drawback can be seen in comparing the scanning process of a traditional CBCT configuration with the one using an offset sensor (FIGS. 2 and 3a to 3c of U.S. Pat. No. 8,363,780). In the traditional configuration, a rotation of 180° allows collecting for each part of the imaged region a complete set of 2D images along the arc. Carrying out the same scanning process along a trajectory of 180° using the offset configuration of FIGS. 3a to 3c reduces the number of 2D images acquired from the peripheral regions, therefore, less information is obtained for the volumetric reconstruction in this region as compared with the traditional configuration of FIG. 2.
In regard to the possible extension of the imaged FOV, the solution according to U.S. Pat. No. 8,363,780, although allowing the acquisition of volumetric images from larger FOV, maintains nevertheless a limit to the FOV dimensions, which depends on the dimensions of the active area of the sensor and the possible maximum offset. Thus in cases where the extended FOV obtained by the sensor's offset is still insufficient for covering the entire region of interest, U.S. Pat. No. 8,363,780 does not suggest a solution for further expanding the FOV so that it covers the region of interest.
The publication “Out-of-core cone beam reconstruction using multiple GPUS” simply suggests how to divide the computational burden of reconstructing the volumetric image out of the set of 2D images acquired during the CBCT scan on several graphic processors, in order to parallelize the reconstruction process that is carried out at the same time on a different part of the acquired image data by each one two or more processing units. This is a conventional processing method which is used in conventional CBCT volumetric imaging for reducing the time needed for the reconstruction of a volumetric image. This document discloses nothing about FOV dimensions and enlargements.
WO2012/139031 discloses generating volumetric images of a large FOV by stitching two dimensional projections. In this document the two dimensional images obtained by the sensor ad related to each of different sensor positions are combined in a bigger 2D image. The volume reconstruction is carried out by using the images obtained by stitching. This method is very sensible to artifact introduced by patient motion or other changes in patient positioning. These changes may occur for each of the 2D images so that the volumetric image is affected by a motion artifact for the entire volume.